drop use python<=3.7 syntax

[kloczek]

Despite what is mentioned in pyproject.toml that sh supports only python 3.8.x sh code still uses older syntax. Pass all code over pyupgrade --py38.

[ecederstrand]

Thanks for the contribution!

black tripped over a single line because it wants double quotes: _unicode_methods = set(dir(''))

If you push that change, then I'll merge.

[kloczek]

One sec I ned to check what failed 😋

[kloczek]

Thank you 😄